ML 1.6 QA: API: Python API coverage

For new public APIs added to MLlib, we need to check the generated HTML doc and compare the Scala & Python versions. We need to track:

• Inconsistency: Do class/method/parameter names match?
• Docs: Is the Python doc missing or just a stub? We want the Python doc to be as complete as the Scala doc.
• API breaking changes: These should be very rare but are occasionally either necessary (intentional) or accidental. These must be recorded and added in the Migration Guide for this release.
  • Note: If the API change is for an Alpha/Experimental/DeveloperApi component, please note that as well.
• Missing classes/methods/parameters: We should create to-do JIRAs for functionality missing from Python, to be added in the next release cycle. Please use a separate JIRA (linked below) for this list of to-do items.